This invention relates to the use of autodepositable aqueous liquid compositions that are both dispersions and solutions in water. By mere contact with these autodepositable liquid compositions, active metal surfaces can be coated with an adherent polymer film that increases in thickness the longer the time of contact, even though the aqueous liquid composition is stable for a long time against spontaneous precipitation or flocculation of any solid phase, in the absence of contact with active metal. (For the purposes of this specification, the term xe2x80x9cactive metalxe2x80x9d is to be understood in its broadest sense as including all metals and alloys more active than hydrogen in the electromotive series, or, in other words, a metal which is thermodynamically capable of dissolving to produce dissolved cations derived from the metal, with accompanying evolution of hydrogen gas, when contacted with an aqueous solution of a non-oxidizing acid in which the activity of hydrogen ions is 1.00 equivalent per liter.) Such liquid compositions are denoted in this specification, and commonly in the art, as xe2x80x9cautodepositionxe2x80x9d or xe2x80x9cautodepositingxe2x80x9d compositions, dispersions, emulsions, suspensions, baths, solutions, or a like term. Autodeposition is often contrasted with electrodeposition, which can produce very similar adherent films but requires that the surface to be coated be connected to a source of direct current electricity for coating to occur.
It is generally believed in the art that autodeposition works because the cations dissolving from the metal surface to be coated are, of course, initially confined to the volume of contacting liquid in the immediate vicinity of the metal surface from which they are dissolving, and these recently dissolved cations interact with the liquid autodepositing composition in at least one of the following ways: (i) The dissolved cations precipitate previously dissolved polymers by displacing previously associated cations or cation-forming moieties, in association with which the polymers are soluble, by the newly dissolved cations in association with which the polymers are much less soluble; and/or (ii) the dissolved cations destabilize numerous individual dispersed phase units in a dispersion of a polymer with inherently low water solubility, which nevertheless can remain in stable suspension for a long time in the absence of dissolved polyvalent cations, because the outer surfaces of the dispersed phase units carry a net negative electrical charge, derived from anionic components of the dispersed polymer itself and/or from an anionic dispersing agent used to prepare the autodepositing composition in question. The net negative charge on the units of the dispersed phase in an autodepositing liquid composition is believed to be electrically counterbalanced by a diffuse excess of cations, usually monovalent cations, in the surrounding continuous phase of the dispersion. This excess of cations together with the negative charges on the dispersed phase units constitutes an example of the well known xe2x80x9celectrical double layerxe2x80x9d or xe2x80x9cHelmholz double layerxe2x80x9d that is characteristic of most interfaces between liquid phases containing charged solute particles and solids in contact with such liquid phases. As long as this double layer remains intact, the net negative charge on the exterior of each unit of the dispersed phase causes it to repel other units of the dispersed phase that also carry a net negative charge, and thereby prevents spontaneous coalescence of the dispersed phase units.
When the double layer is sufficiently disturbed (or in the case of a soluble polymer, when the solubility is reduced) by introduction of new cations, the polymeric parts of numerous dispersed phase units and/or solute polymer molecules aggregate. Aggregated polymer molecules, along with some of the liquid in which they were dissolved or dispersed, form initially a wet coating layer with at least sufficient cohesion to resist completely draining away under the influence of the Earth""s gravity. Upon further drying, this wet coating layer forms a continuous dry solid film, if the chemical nature of the polymer favors such a transition and the temperature during drying is sufficiently far above the glass transition temperature of the polymer concerned. Polymers that have this property of forming a continuous solid film or body from initially finely dispersed or dissolved particles of the polymer in a liquid dispersion medium or solvent are defined as xe2x80x9cfilm-formingxe2x80x9d polymers, and at least one such polymer is a necessary constituent of every conventional autodeposition bath. The continuous solid film formed by the film-forming polymer content of an autodeposition bath may constitute the entire solidified and cured autodeposited coating, and any instance normally constitutes the only continuous solid phase of the solidified and cured autodeposited coating. It is quite common for a cured autodeposited coating also to include at least one discontinuous phase, most often a pigment such as carbon black.
In other kinds of polymers, both coatings and solid objects, it is common to include a discontinuous solid phase, usually called xe2x80x9cfillerxe2x80x9d or xe2x80x9creinforcementxe2x80x9d, that is dispersed in the continuous polymer phase and acts to alter the properties of the composite formed by the continuous and discontinuous solid phases from those that prevail in the continuous phase only. In many instances, it is possible both to reduce the cost per unit volume and to increase the mechanical strength by including in a polymer a mineral filler such as clay, calcium carbonate, or the like. Although the inclusion of such materials in an autodeposited coating has been taught in prior patents, it is not believed that any use of dispersed solid phases in autodeposition baths has attained profitably practical success for any purpose except pigmentation. The use of two or more types of film-forming resins in an autodeposition bath has also been taught, but again is not believed to have achieved profitably practical success.
One major object of this invention is to provide autodepositing liquid compositions from which it is possible to deposit on contacted metal surfaces coatings that contain dispersed solid phases, which provide for autodeposited coatings at least some of the advantages practically achieved in other uses of polymers by the inclusion of fillers and/or by the use of composite structures containing more than one type of polymer. Other alternative and/or concurrent objects will be apparent from the further description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight or mass; the term xe2x80x9cpolymerxe2x80x9d includes xe2x80x9coligomerxe2x80x9d, xe2x80x9ccopolymerxe2x80x9d, xe2x80x9cterpolymerxe2x80x9d and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word xe2x80x9cmolexe2x80x9d means xe2x80x9cgram molexe2x80x9d, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms xe2x80x9csolutionxe2x80x9d, xe2x80x9csolublexe2x80x9d, xe2x80x9chomogeneousxe2x80x9d, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25xc2x0 C.; and the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation.
It has been found that at least one of the major objects of the invention can be achieved by including in an autodeposition bath a suitable quantity of non-film-forming dispersed organic material that codeposits with the film-forming-resin(s) also present in the autodeposition bath into the wet autodeposited film.